Radio frequency (RF) power amplifiers (PAs) are used extensively in the wireless communications industry. An example of an RF PA that is suitable for use in, for example, a transmitter of a cellular telephone or wireless personal digital assistant (PDA), is shown in FIG. 1. RF PA 10 includes an amplifier circuit 12 having multiple amplifier stages. Multiple amplifier stages are used to achieve an overall power gain that may not be realizable using only a single stage at the RF frequencies of interest. In this manner, power amplifier circuit 12 provides successively increasing signal powers. In RF PA 10 in FIG. 1, amplifier circuit 12 is shown to contain three amplifier stages—a first stage 14, an intermediate stage 16, and a final stage 18. Typically, coupling capacitors 20 are connected to the inputs of each stage 14, 16 and 18. Each coupling capacitor 20 couples an RF input signal to the input of the associated amplifier stage and blocks DC components that may be present with the RF input signals.
RF PA 10 also includes a power distribution network 26, which distributes power to amplifier circuit 12. Typically, an operating supply VOP is distributed by power distribution network 26 to provide power to first 14 and intermediate 16 stages, and a separate power amplifying power supply VPA is distributed by power distribution network 26 to provide power to final stage 18. Final stage 18 is often powered separately from first 14 and intermediate 16 stages, since it typically has a power requirement that is substantially greater than power demands of previous stages. VOP is distributed to first 14 and intermediate 16 stages through inductors 28, and VPA is distributed to final stage 18 through inductor 29. Inductors 28 and 29 present high impedance paths to RF signals and low impedance paths to DC power supplied by VOP and VPA.
As shown in FIG. 1, one or more RF bypass capacitors 30 are connected to operating supply VOP, and an RF bypass capacitor 31 is connected to power amplifying power supply VPA. Inclusion of the bypass capacitors 30 and 31 ensures that the AC impedances between VOP and ground, and VPA and ground, are nearly zero. These AC grounds are essential at the intended operating frequencies of the RF PA 10, since the entire signal voltage must appear across the inductors 28. At operating frequencies typical of RF amplifiers used in cellular systems, the values of these bypass capacitors are typically between 10 and 30 pF.
At frequencies lower than the intended amplifier operating frequency, the impedances of bypass capacitors 30 and 31 increase and bypass capacitors 30 and 31 become less effective at damping low frequency oscillations and providing AC grounds between VOP and VPA. Also at lower frequencies, the gains of one or more of the amplifier stages 14, 16 and 18 also typically increase. An increased gain at lower frequencies is undesirable, since it may lead to low frequency oscillations and amplifier instability. Accordingly, to prevent low frequency oscillations and amplifier instability, additional bypass capacitors 34 and 36 are usually connected between operating supply VOP and ground, and between amplifying power supply VPA and ground. Compared to bypass capacitors 30 and 31, low frequency bypass capacitors 34 and 36 are quite large, and typically have capacitances in the range of 10,000 pF to 10 μF, or greater.
RF PA 10 further includes a biasing network 38 having one or more active bias circuits 39. Active bias circuits 39 set operating conditions for the active devices in each of stages 14, 16 and 18 of amplifier circuit 12. While setting the threshold conditions, active bias circuits 39 compensate for variations and/or instabilities in gain of the active devices due to shifts in operating conditions caused by various effects, such as variations in temperature among devices and physical differences among devices due to manufacturing differences.
In a conventional quadrature (IQ) modulator, a fully amplitude and phase modulated signal is applied to the RF input 22 of RF PA 10, and amplifier circuit 12 is biased by active bias network 38 for linear operation. A major problem with the conventional RF PA 10 design in FIG. 1, however, is that it does not function properly for polar modulation. Polar modulation is a modulation technique that splits an RF signal into two separate signal paths, one path that transmits an amplitude modulated signal and the other that transmits a phase modulated signal. Unlike IQ modulation, with polar modulation only a constant magnitude, phase modulated signal is applied to RF input 22 of RF PA 10. The phase modulated signal is used to control an on-channel voltage controlled oscillator (VCO), which provides a drive signal for driving the gate of a FET (or base, if a BJT) in first stage 14 of amplifier circuit 12. The amplitude modulated signal is used to modulate the drain (or collector, if a BJT) supply voltages VPA and/or VOP. Although phase and amplitude information are independently processed, they are digitally synchronized so that the attributes of the unamplified input signal may be recovered. Further details of polar modulation can be found in U.S. Pat. No. 6,366,177 to McCune et al. and M. Heimbach, Digital Multimode Technology Redefines the Nature of RF Transmission, Applied Microwave & Wireless, August 2001.
As alluded to in the previous paragraph, envelope variation of one or both of the supply voltages VPA and VOP in a polar modulation approach is achieved by varying the amplitude of the supply voltage in a respective stage, synchronous to the already applied phase modulation. Depending on the RF technology employed, the frequency of the drain modulation of supply voltages VPA and/or VOP can be quite high. For example, for EDGE (Enhanced Data GSM (Global System for Global Communications) technology, the rate is on the order of 1 MHz. For UMTS (Universal Mobile Telecommunications System), rate rises to the order of 10 MHz. Accordingly, for polar modulation to be realizable, power distribution network 26 of RF PA 10 must be capable of tracking rapid variations in VOP and/or VPA. Unfortunately, particularly for EDGE and UMTS technologies, power distribution network 26 is incapable of tracking such rapid variations. A primary reason for this is attributable to low frequency bypass capacitors 34 and 36, which inhibit the ability to rapidly change the supply voltages VOP and/or VPA due to their large capacitances.
Another problem associated with the conventional RF PA 10 design shown in FIG. 1 involves the propagation, i.e., undesired “feedback” of RF signals from an input of one stage, 14, 16 or 18, through the biasing network 38, to an input of another stage. This undesired coupling can result in distortion of the RF signal provided at the RF output 24 of RF PA 10 and instability in the gain of amplifier circuit 12.
The inability to track and provide rapid variations in supply voltage to amplifier circuit 12, and the instability problems attributable to RF coupling on the biasing network 38, render the RF PA design 10 in FIG. 1 not practicable for polar modulation systems. Accordingly, there is a need for an improved RF PA that is capable of supporting polar modulation.